The present invention relates generally to the field of cathodes and rechargeable electrochemical cells. More particularly, the present invention pertains to solid composite cathodes which comprise: (a) an electroactive sulfur-containing material; (b) a crosslinked polymer formed from a reaction of a polymeric material having carboxyl groups and a crosslinking agent; and (c) a conductive filler. This crosslinked polymer improves the flexibility and adhesion of the cathode, and the cycle life of the cell.
Throughout this application, various publications, patents, and published patent applications are referred to by an identifying citation. The disclosures of the publications, patents, and published patent applications referenced in this application are hereby incorporated by reference into the present disclosure to more fully describe the state of the art to which this invention pertains.
As consumer demand for, and reliance upon, portable and hand-held electronic devices such as mobile telephones, portable computers, pagers, and palm pilots has grown, so has the need for portable power supplies, such as rechargeable batteries, with long cycle life, rapid recharge capacity, and high energy density, become more important. There has been considerable interest in recent years in developing high energy density primary and secondary batteries with alkali-metal anode materials, and, in particular, anodes based on lithium.
Various types of cathode materials for the manufacture of thin film alkali-metal batteries are known in the art.
Elemental sulfur is an attractive cathode material in alkali-metal batteries owing to its low equivalent weight, low cost, and low toxicity. Many alkali-metal/sulfur cells have been described, as for example, in U.S. Pat. Nos. 3,532,543, 3,953,231, and 4,469,761; Rauh et al., J. Electrochem. Soc., 1979, 126, 523-527; Yamin et al., J. Electrochem. Soc., 1988, 135, 1045-1048; and Peled et al., J. Power Sources, 1989, 26, 269-271.
In spite of the many known systems, solid composite cathodes comprising elemental sulfur in rechargeable alkali metal sulfur battery systems have been problematic in obtaining good electrochemical efficiency, utilization, capacity, cycle life, and safety of the cells owing to the diffusion of sulfur-containing active materials from the sulfur-containing cathode into the electrolyte and other components of the electrochemical cells. This has been particularly true in electrochemical cells comprising a sulfur-containing cathode in combination with a lithium-containing anode.
One approach to addressing these problems has been through the use of polymeric binders in the composite cathodes. These polymeric binders fall generally into three types: (a) polymer electrolyte ionically conductive polymers; (b) electronically conductive polymers; and (c) non-ionically conductive and non-electronically conductive polymers, often referred to as inert binders.
Examples of polymer electrolyte polymers used in sulfur-containing solid composite cathodes include polyethylene oxides, polypropylene oxides, polyacrylonitriles, polysiloxanes, polyimides, polyethers, sulfonated polyimides, divinyl polyethylene glycols, polyethylene glycol-bis acrylates, polyethylene glycol-bis methacrylates, polyphosphazenes, and polyether grafted polysiloxanes, as for example described in U.S. Pat. No. 4,303,748 to Armand et al.; in U.S. Pat. Nos. 5,529,860 and 5,690,702 to Skotheim et al.; and, in U.S. Pat. No. 5,686,201 to Chu. These polymer electrolyte polymers have ionically conductive properties which may improve the electrochemical utilization, efficiency, and capacity upon cycling of the cell.
Examples of electronically conductive polymers used in sulfur-containing solid composite cathodes include polyanilines, polyacetylenes, polypyrroles, polythiophenes, polyphenylene-vinylenes, polythienylene-vinylenes, and polyphenylenes, as for example described in U.S. Pat. Nos. 5,529,860 and 5,690,702 to Skotheim et al. and in U.S. Pat. No. 5,686,201 to Chu. These electronically conductive polymers provide electrical conductivity to the solid composite cathodes and may have electrocatalytic properties to improve the electrochemical utilization, efficiency, and capacity of the cell.
Examples of inert binders used in sulfur-containing solid composite cathodes include polytetrafluoroethylenes and other fluorinated polymers, styrene butadiene rubbers, and ethylene-propylene-diene rubbers, as for example described in U.S. Pat. Nos. 5,529,860 and 5,690,702 to Skotheim et al. These inert binders typically provide mechanical strength and improved cohesion and adhesion to the solid composite cathode layer. In U.S. Pat. No. 5,846,674, to Sakai et al., reaction-curing binders for lithium transition metal oxide electrode active materials, by heat and/or radiation curing, possessing enhanced adhesion to metal foil current collectors in non-aqueous cells, are described. Examples of reaction curing binders include urethane oligomers and acrylates.
Another type of polymeric binder are cationic polymers, as described in U.S. Pat. No. 6,110,619 to Zhang et al., that improve the electrochemical utilization and cycling efficiency of sulfur-containing electroactive materials when incorporated into the cathode of an electrochemical cell.
An alternative method of preparing the sulfur-containing solid composite cathodes comprises heating an electroactive sulfur-containing material to a temperature above its melting point and then resolidifying the melted electroactive sulfur-containing material to form a cathode active layer having redistributed sulfur-containing material of higher volumetric density than before the melting process, for example, as described in PCT Publication No. WO 00/36674 to Xu et al.
Despite these approaches there still remains a need for improved binders which provide in sulfur-containing solid composite cathodes a combination of mechanical strength, flexibility, adhesion, coatability, increased electrochemical utilization, efficiency, and capacity in the cycling of the electroactive cathode material, and inertness to chemical reaction with the electroactive cathode material and other components of the cell, such as liquid electrolytes, that may contact the polymeric binder.
The solid composite cathode of the present invention for use in an electrochemical cell comprises: (a) an electroactive sulfur-containing material; (b) a crosslinked polymer formed from a reaction of a polymeric material having carboxyl groups and a crosslinking agent; and (c) a conductive filler.
In one embodiment of the present invention, the polymeric material having carboxyl groups is selected from the group consisting of polymers and copolymers of acrylic acid, methacrylic acid, crotonic acid, fumaric acid, and maleic acid. In a preferred embodiment, the polymeric material having carboxyl groups is a copolymer of acrylic acid selected from the group consisting of ethyl acrylate-acrylic acid copolymers, butyl acrylate-acrylic acid copolymers, vinyl pyrrolidone-acrylic acid copolymers, ethylene-acrylic acid copolymers, and vinyl acetate-acrylic acid copolymers. In another preferred embodiment, the polymeric material having carboxyl groups is a copolymer of crotonic acid.
Suitable crosslinking agents include, but are not limited to, those selected from the group consisting of aziridines, phenolic resins, and melamine resins. In one embodiment, the aziridine is selected from the group consisting of trimethylol propane tris [xcex2-(N-2-methyl aziridinyl)propionate], trimethylol propane tris [xcex2-(N-aziridinyl)propionate], and pentaerythritol tris [xcex2-(N-aziridinyl)propionate].
Suitable conductive fillers include, but are not limited to, those selected from the group consisting of carbon black, graphites, activated carbon fibers, non-activated carbon nanofibers, metal flakes, metal powders, and electrically conductive polymers.
In one embodiment of the present invention, the solid composite cathode comprises 60 to 85% by weight of an electroactive sulfur containing material. In one embodiment of the present invention, the solid composite cathode comprises 2 to 10% by weight of the crosslinked polymer. In one embodiment of the present invention, the solid composite cathode comprises 15 to 35% by weight of a conductive filler.
In one embodiment, the electroactive sulfur containing material comprises elemental sulfur. In one embodiment, the electroactive sulfur containing material, in its oxidized state, comprises a polysulfide moiety of the formula, Sm, wherein m is an integer equal to or greater than 3. In yet another embodiment, the electroactive sulfur containing material comprises a sulfur-containing organic polymer.
Another aspect of the present invention pertains to methods of preparing a solid composite cathode comprising the steps of: (a) dispersing or dissolving, in a liquid medium, an electroactive sulfur-containing material, a polymeric material having carboxyl groups, a crosslinking agent, and a conductive filler; (b) casting the composition resulting from step (a) onto a suitable substrate; and (c) removing some or all of the liquid from the composition resulting from step (b) and crosslinking the polymeric material and the crosslinking agent to provide a solid composite cathode.
In one embodiment, the substrate is a metallized plastic film. In one embodiment, the metallized plastic film is less than 15 microns in thickness.
Another aspect of the invention pertains to an electrochemical cell comprising an anode comprising lithium and a composite cathode, as described herein.